It is known to measure the level of a liquid by the use of reed switches which are actuated by a float containing a permanent magnet as disclosed in U.S. Pat. No. 4,976,146.
Such system uses a plurality of reed switches. In the event that one of the reed switches fails the sensor is unable to perform its measuring function. This becomes a severe limitation in level sensors having a multitude of sensors, for example, as great as 1040 sensors, wherein the likelihood of a failure becomes greater. Secondly, the prior art system utilized a decade interconnection system. While this method reduced the number of electrical components, it still used ten wires from the first ten unit switches in numerical order each connected to a set of ten wires from each of the tens switches connecting all of the tens, and finally connecting all of the 100s to ten wires. This resulted in a requirement of 31 wires including a common wire exiting the level sensor. This multitude of wires, due to decreased conductor spacing, increase the likelihood of transients and lightening faults and limited the measurement range. That is even with 31 wires the sensor was limited to 1000 measurement positions.
The present invention is directed to a digital read out of a reed switch liquid measuring sensor through shift registers, allowing each individual reed switch to be addressed to determine if it is open or closed, and incorporates a microprocessor connected to the sensor to minimize the electrical connections from the sensor, which allows multiple floats to be operated on the same sensor for measuring multiple liquids having different densities, and provides a method of operation so that any failed switches are identified and ignored thereby providing a sensor which continues to function even with failed open or closed reed switches.